FIG. 2A shows a simplified block diagram of a typical trunked radio system 200. In such systems, access to communication resources, for example voice channels, is controlled by a central resource controller, for example 201. This resource controller is usually coupled, for example via wireline, to a plurality of repeaters such as 203, 205, and 207. Resources, or channels, are requested through a dedicated repeater acting as a control resource, for example 203. When a resource is requested by a communication unit, such as portables 209 or mobiles 211, an available channel 215 will be chosen and assigned via a channel assignment to that unit. Today, this is generally accomplished by transmitting data via a control resource or channel. An example of such a transmission is given in FIG. 2A and indicated by transmission 213.
If a communication resource is not available, a "busy" will be issued and repeated on the control channel until the time that a suitable voice channel becomes available and an assignment can be issued. Once a busy is issued to, and received by, a communication unit, that particular unit cannot leave the control channel to be involved in any other call as it may then miss the assignment for the call originally requested. These calls may be a variety of types, for example group dispatch, individual dispatch, or telephone interconnect calls. The aforementioned voice channel requests, busies, and channel assignment grants are accomplished by repetitious signals being transmitted on the control resource, or control channel. These transmissions on the control resource are known in the art as inbound signalling words (ISW), which originate at the communication units, and outbound signalling words (OSW), which originate at the resource controller.
In the arena of radio frequency (RF) communication, trunked systems can be found in all different sizes. A smaller system may have as few as two repeaters, one of which may be used for voice/data transmission and the other remaining channel may be dedicated to the transmission of control signals, such as OSWs and ISWs. Large scale trunked communication systems, on the other hand, may have as many as 28 channels, 27 of which may be used for voice/data transmission and the remaining channel used for transmitting control signals. In a larger system, such as the one described having 28 channels, there may be as many as two thousand communication units being served on that system. It is not difficult to imagine the control traffic congestion that occurs when even a fraction of these radios are requesting access to a voice channel for communication.
In today's trunked communication systems, a busied communication unit, that is, a subscriber unit whose channel request has been denied until a channel of the type requested becomes available, is "locked-in" on the control channel until his request is granted, or he cancels his request. For a group dispatch call, these busies may typically last anywhere from one-half second to eight seconds, depending on the traffic load of the system at the time of the request. The actual call, defined as that which begins with the pressing of the push-to-talk (PTT) button and ending with the release of that button, typically last on the order of twenty seconds. For a telephone interconnect call, which may last anywhere from five minutes to 30 minutes, the busy duration is typically between one and four minutes. It is not difficult to see that the typical duration of a busy can be a significant portion of the total transmission time for a given call type. In a worst case scenario, the actual busy duration may exceed the total call duration. This is often the case on a heavily loaded system having only one control resource, or channel.
Accordingly, there exists a need for a trunked communication system that can, through the use of reduced OSW traffic and enhanced subscriber unit channel assignment methods, make efficient use of limited resources available during times of heavy usage.